Widely available at office supply stores, the pen contains a solution that detects bills printed on common copier paper rather than the cotton-and-linen-fiber paper the U.S. government uses.

When a librarian marks a bill with the pen and the mark turns black, that means the bill is bogus. If the mark turns yellow or remains clear, the money is good.

For about a month, Houston’s libraries have been checking all bills larger than $1, said spokeswoman Sandra Fernandez.

“All of our libraries had already been checking all bills $20 and above,” Fernandez said. “We just recently started checking lower bills.”

The library’s Financial Services Division decided to change the policy after the discovery of some phony small bills.

“We took a step back and decided to check lower denomination bills,” she said.

Although the library doesn’t deal with a lot of cash, it takes in money for overdue-book fines and book sales, she said. Under the new policy, those bills must be examined.

“We’re sorry for the inconvenience to our customers,” Fernandez said. “We have to make sure the impact to us and to them is minimal.”

Whenever fake money shows up at the library, its Financial Services Team must give the bills to the library’s bank, which then files a report with the U.S. Secret Service.

Although not high-tech, the pen is a useful tool for libraries to screen incoming cash, said Cynthia Marble, the Secret Service special agent in charge in Houston.

“It’s certainly a first line of defense for them to check something like that,” Marble said. “But that doesn’t mean it’s 100 percent foolproof.”

For example, it won’t catch the counterfeiter who turns a genuine $1 bill into an illegal $20.

Authorities haven’t seen an upsurge in counterfeit small bills in Houston, Marble said.

Although would-be counterfeiters have increasingly sophisticated photocopiers at their disposal, their efforts don’t always match the technology, Marble said.

“It’s easier to produce, but the quality is not all that good,” she said. “Merchants and banks are very astute. They handle it all the time. They know what they’re seeing, what they’re feeling.”

Thankfully, there doesn’t seem to be a trend of counterfeiters trying to unload small bills at public libraries, said Marcia Warner, president of the Public Library Association, based in Chicago.

Houston’s recent policy of checking even $5 bills is the first such move she had heard about, said Warner, who has been a library director for 34 years in Michigan. “We’re pretty trusting as a profession,” she said. “I don’t think it’s a widespread issue.”

The U.S. Secret Service offers these tips on detecting counterfeit bills:

• On a genuine bill, the portrait is lifelike and stands out from the background. The counterfeit portrait is usually lifeless and flat.

• On a real bill, the Federal Reserve and Treasury seals’ saw-tooth points are clear and sharp. Counterfeit seals may have uneven or broken points.

• The fine lines in a genuine bill’s border are clear and unbroken. On the counterfeit, lines in the outer margin and scrollwork may be indistinct.

• Genuine serial numbers have a distinctive style, are evenly spaced and are the same color as the treasury seal. Counterfeit serial numbers may differ in ink color from the seal, and numbers may be unevenly spaced.

• Genuine currency paper has tiny red and blue fibers embedded throughout. Counterfeiters often try to simulate these fibers by printing red and blue lines on the paper.